Multi-column gas chromatography for analysis of a formation sample

A method comprising:

• injecting, into a gas chromatography instrument, a gas sample having a number of chemical compounds;
• separating out, by a primary column, a hydrocarbon subset that includes hydrocarbons from the number of chemical compounds;
• analyzing, by a primary detector, the hydrocarbon subset for hydrocarbons having a weight that is less than a lower weight threshold;
• determining whether at least one trigger event that is associated with gas sample has occurred;
• and
• in response to determining that the at least one trigger event occurred,
• separating out, by a secondary column, at least one isomer from the hydrocarbon subset;
• and
• analyzing, by a secondary detector, the at least one isomer separated out by the secondary column.

A system comprising:

• a gas separator, during drilling of a wellbore into a subsurface formation by a drill string, to receive a formation sample from the subsurface formation, wherein the gas separator is to separate out a gas sample from the formation sample;
• a gas chromatography instrument comprising:
• a sample injector configured to inject the gas sample that includes a number of chemical compounds;
• a primary column to separate out a hydrocarbon subset that includes hydrocarbons from the number of chemical compounds;
• a secondary column valve coupled to an output of the primary column;
• a primary detector coupled to a first output of the secondary column valve and configured to analyze the hydrocarbons of the hydrocarbon subset;
• a heart cutting unit coupled to a second output of the secondary column valve, the heart cutting unit configured to cut, from the hydrocarbon subset, isomers having a weight greater than a threshold;
• a secondary column coupled to an output of the heart cutting unit, the secondary column configured to separate out at least one isomer from the cut isomers;
• and
• a secondary detector configured to analyze the at least one isomer separated out;
• and
• a controller to control the secondary column valve such that the controller is to open the secondary column valve to output the hydrocarbons to the heart cutting unit in response to at least one trigger event having occurred, wherein the controller is configured to transmit a communication to a device to cause the device to perform or adjust the drilling, based on analysis of at least one of the hydrocarbon subset and the isomers separated out.